The Kakuja Effect
by Star-Nebula42
Summary: The changes have started. The first thing to change was his sense of smell. The second one was his taste. The third one was the insatiable hunger. The forth was his attraction to people, not in the romantic sense, but the alluring smell of them. Rin disappeared during the night for nearly two years. Out of nowhere he returns, but as a cold-blooded killer. (Slight AU!)
1. Prologue: The Kakuja Effect

**Title:** The Kakuja Effect: Prologue

 **Chapter:** 1/?

 **Info:** This story starts around the True Cross Academy Festival arc. In this AU Shima is not a spy. This story was inspired by Tokyo Ghoul, in which there are ghouls (ghouls are similar to demons), who mutate through repeated canibalization.

 **Disclaimer:** I don't own Ao no Exocrist/ Blue Exorcist or any of it's characters.

* * *

The changes in Rin were subtle at first. By the time he noticed something was wrong, it was already too late.

First, his sense of smell changed. The smell of freshly cooked Sukiyaki, his favorite meal, instantly made him nauseous. It appealed to him as much as fish intestines that had been left out in the hot sun to rot.

After his sense of smell changed, his sense of taste followed soon after. Regardless of what he ate, or what he cooked, everything he ate tasted like it was on the verge of spoiling. Even just looking at food made him feel sick to his stomach.

Thirdly, and in his own opinion, the most bizarre thing to change was his attraction to people. Not in the romantic sense, but the alluring smell of them. To him, they smelled sweet like freshly baked sticky buns.

Forth was his constant and insatiable hunger that kept him awake at night. His hunger was constantly in the front of his mind, but no matter what he ate, it left him feeling sick and sequentially would lead to him vomiting the contents of his stomach. The painful ache of his empty stomach was getting worse by the hour.

Yukio left the dorm room earlier that morning, he mentioned something about a low-level demon that was running amok and causing the order some trouble. Yukio glanced over at his older brother and realized that he was still sound asleep. He left the room without another word.

' _Some things never change._ ' Yukio thought as he left the room.

Rin waited, completely still and quiet for the door to close. When he heard the sound of the latch click shut, he unintentionally let out a small whimper. He hadn't been sleeping at all. The sweet taste of his blood coated his lips and tongue, to keep quiet, he had bitten down on his hand to keep himself from crying out and alerting Yukio.

Warm tears ran down his face, God, he was so hungry. What the hell was going on with him? Everything he tried to eat made him feel sick. He raked a hand anxiously through his hair, he had no idea what was going on.

"Please stop..." He cried, "Someone please, help..." But Rin was sat completely alone in his room, leaving no one to hear his cried for help.

 _'Yukio, he would know what to do, right?'_ Rin thought desperately, _'No! Don't cause him any more trouble! You've caused him enough problems as it is.'_

Shakily, he pushed himself into a seated position and pulled his knees tightly to his chest. He rested his head on his knees and sat there for a long while. Reading the clock on the desk, a bolt of shock jolted through him. An hour had already passes since Yukio had left, even though it felt like he was just there moments ago.

Gripping the desk to stabilize himself, he slowly stood up. Malnutrition from a lack of nutrients made his legs shake with fatigue. He stood there for a moment and tried to pull himself together. He couldn't let anyone know. He couldn't look weak in front of them. They would call him pathetic.

Deep down, Rin knew that wasn't true. His friends would stand by his side and help him, but his twisted, ravenous thoughts told him other wise, coaxing him into believing that this was all his fault.

"Just make it through today." Rin begged himself.

-~x~-

Yukio was a patience person by nature, especially when it came to Rin. But sometimes Rin really liked to put his patience to the limit. Yukio had waited an extra five minutes to start his class. Then ten. And then fifteen. It became clear that Rin had every intention on missing class, and absolutely no intentions on joining his classmates for class. Even Yukio couldn't stop the world for his brother.

Yukio rubbed at his temples, fending off the impending headache that was building behind his eyes. It would take a little more than a tardy student, brother or not, to break his composure.

Yukio let out a long sigh before apologizing to the exwires, and started his lecture on how to properly clean Naberious inflicted wounds and how to stop them from becoming infected.

He was part way through handing out a list of purifying herbs that we commonly used to help reduce the swelling of Naberious wounds when the classroom door creaked open.

Without looking up from what he was doing, he simply said, "Rin, take a seat next by Miss Moriyama. Shiemi, could you please fill him in on what we just went over-" He looked up at his older brother and froze.

Rin was a complete mess. Aside from being utterly disheveled, his eyes, red and swollen as though he had spent the last hour crying. His eyes, normally so full of joy, were void of their fiery passion. He had a fake smile plastered on to his face, and waved at the class.

"Sorry I'm late!" He said with forced cheeriness, "It won't happen again."

Before Yukio could collect his thoughts, Rin shuffled over to his desk and slumped into the chair. tossing his bag half-heartedly on the ground. The whole classroom had gone silent.

Yukio walked over and leaned towards his brother, just out of earshot of the others and whispered, "Nii-san, what's wrong?"

"Nothing." Rin replied too quickly, and then added, "Nothing. I promise."

Rin avoided making eye contact with Yukio.

Yukio gave him a pointed look, his face was full of concern.

"R-right, as I was saying..."

He handed Rin a list of the herbs they were going over and continued his lecture.

He spared a glance at his brother, Shiemi was trying to explain to Rin what he had missed out on, but Rin looked vacantly ahead of him. Clearly lost in another world. Only nodding and asking 'Really?' when Shiemi paused.

Yukio's eyes wandered around the classroom and he caught Bon's eyes, his troubled expression mirrored his own.

Something was wrong.

At the end of class Yukio grabbed his brothers elbow and pulled him to the side.

"Rin, what's going on? Is everything okay?" He asked, his face full of worry.

"It's nothing." Rin replied easily.

Yukio sensed the lie. "Are you sur-"

"It's nothing" he hissed back at Yukio. Rin wrenched his elbow from Yukio's hand and walked swiftly from the classroom.

Yukio stood there, stunned. His mind working at one hundred miles a minute trying to understand what just happened. He would have to ask him later tonight at the dorm room, where they could talk in private.

The only issue was that Rin didn't return to the dormitory that night.

And he didn't return for two more years.


	2. In the Shadows

**Title** : In the Shadows

 **Rating** : T+

 **Word count** : 1,418

 **Chapter** : 2/?

 **Authors note** : Thank you so much for reviewing! I look forward to reading your reviews :) I'm aiming to update this once a week but please expect some bi-weekly updates.

 **Disclaimer:** I do not own Ao no exorcist or any of the characters.

* * *

That night he dreamt about a life forgotten. No that's wrong. Not forgotten, abandoned. It was on days like this where his dreams were more desirable than reality. Sweet dreams of going to sleep without waking up hungrier than the night before. Waking in a warm bed with a little furry cat keeping his legs warm. Kuro.. Oh how he missed that demon.

The cot creaked as Rin groggily sat up, wiping the sleep from his eyes. He began walking over to his dresser The floorboards creaking underneath him as he did so. Throwing on a long coat he walked out the front door, not even bothering to fix his disheveled hair. He threw his hood up as he walked out. It had been rainy the past few days and rain meant fewer people were out but, there was never any harm in being cautious.

He needed to find something to eat before the end of the week. The weekends were far too crowded in Tokyo to snag a meal without someone sparing him a second glance.

He stood at the end of a narrow alleyway, watching. His eyes narrowing in concentration waiting for an opportunity. It was, admittedly, more crowded than he hand anticipated. Staying close to the wall he watched as people passed by. Men in business suits rushing to work to finish that last email before the weekend. Mothers using their children along in hopes of getting them to school on time. Teenagers reeked of alcohol causing them to slur as they howled with laughter.

As the lights turned green signaling it was safe to cross the street an opportunity presented itself. A women. He glanced around. She was alone. Her sweet scent flooding his senses. Her warm, sweet blood pulsing through her veins. She was on the phone and distracted. Perfect.

As she passed the opening she was abruptly pulled into the alleyway. She tried screaming but, a cold hand covered her mouth. She tried to bite it only to have her hair pulled, yanking her neck back in a harsh way.

"Shhh… I wouldn't do that if I were you." Her attacker said in a spine-chilling voice. She desperately tried to overwhelm her attacker but failed miserably. His grip around her was too strong. If only she could reach the knife in her coat pocket without making a perilous mistake.

While he's distracted watching for passerby, she gave a violent lurched catching her attacker by surprise and plunged her hand into her pocket grasping her fist firmly around the knife. Without hesitation she ripped the knife into the mans ribs and ran for it.

Right into the street into oncoming traffic.

He heard the squealing of breaks followed by a brief bloodcurdling scream and the horrific sound of bones being broken in a hundred, no, a thousand different ways. The sound would be engraved in his memory for years to come.

"Shit." He swore. Well, at least she couldn't run to the police. Now that the only witness of what he had just attempted was dead.

He let out a chuckle. "Well.." He began "that was certainly unexpected. Though.. It was entertaining." He finished morbidly. Sighing as he realized he still needed to find a meal. Shoving his hands in his pocket he whistled an unknown tune and walked back into the ally.

* * *

"We have been receiving reports of missing people." Arthur spoke with a booming voice unnecessarily as he and a few select others were sitting around a small table. "Now as you all know, we don't take 'just' reports of missing people." Emphasizing on the word 'just'.

"We believe it is a high level demon causing these disappearances. And I, the paladin, plan to find out. Now if you would, take a look at the folders you've been presented with."

Yukio, who was the youngest exorcist present, grimaced slightly. The photos inside the folders were truly grotesque. Just looking at them caused his stomach to churn.

He looked at them as quickly as possible.

Bodies. A series of bodies that had been viciously ripped apart. The top photo alone was what appeared to be the remains of a young adult male. Blood sprayed on the walls, pooling at the photographers feet. Chunks of gory flesh was all that remained. The attacker had been unmerciful.

Yukio looked away from the photos and tuned back into the conversation ".. And if you'd take a look at the last photo it's the only one that we have of the.." He hesitated, trying to pick the most fitting name "Monster. It's not much to go on. It appears to have a humanoid silhouette."

His mind immediately jumped to Rin. 'Don't be stupid. It's not him. He's gone.' Within two days of Rin not showing up to any of his classes or back at the dormitory there was a search team created. Yukio being a part of it of course. After three months of searching much to Yukio's dismay, the search was finally called off. When he informed the Cram school students that Rin would not be returning everyone felt devastated.

He pushed those thoughts from his head. Rin wouldn't want Yukio to live in the past. "We are defined by our futures, not our past." Rin once said. Yukio smiled at the memory.

The picture wasn't much to go on. It appeared that it was shot on a low quality security camera.

All that could be made out was a back figure hunched over a body.

"Rumors are that this demon is a Kakuja. For those who don't know a Kak-"

"I thought they were just a myth." Someone interrupted.

"Ahem, if you would let me finish. They were believed a myth. From recent studies we were able to recreate the process. A Kakuja isn't a type of demon per se but, actually a rare mutation that can occur from repetitive cannibalization." The group felt a slight chill. These demons weren't to be messed with. "Now, this demon in particular has no distinct pattern, most demons fall into a pattern. A lazy habit. The odd thing about this one is it does not limit itself to only eating demons to grow stronger but also humans. We assume it kills the latter is for the thrill of the hunt."

He cleared his throat and continued "It has killed countless demons but we aren't consider about that. What we're concerned about are the 48 human deaths linked to this particular demon. We aren't sure if it's connected but, just last week there was a women killed in a supposed traffic incident. Witnesses say that she suddenly ran out into the middle of the road. Now while this in itself may not seem connected, it was in the same area that we believe the demon resides, Shibuya Tokyo. We leave in three day's time."

It was dark outside by the time Yukio left the meeting. With so little information to go off of, the mission could be very dangerous. The demon has the element of surprise. While mauling these thoughts over, Yukio deemed that a trip to the library would be beneficial. It would certainly help in preparation for the upcoming mission. With a rare demon like this he would need all the information he could get.

Walking down one of the aisle a book caught his eye. 'Demonic Attributes: Uses and Causes.'

He skimmed through the pages until a passage caught his eye.

".. Although most Kakujas are self-created through repetitive cannibalization, there are some who are born with a rare mutated gene. Occasionally this gene won't surface until the demon reaches full skeletal and/or sexual maturity. The last known sighting of a Kakuja was in 1958 when a young European girl.."

Yukio closed the book and tucked it under his arm. This book could prove useful.

He sat down at a nearby desk with a pad and pencil and began taking notes. The ticking of a clock on a nearby wall echoed through the library.


	3. Dreamless-Nightmares

**Title** : Dreamless-Nightmares

 **Chapter** : 3/?

 **Authors note:** Thank you guys so much for following, favoriting and reviewing! Also- Thank you FancyUsername.

 **Disclaimer** : I do not own Ao no exorcist or any of the characters.

 **Q** : How does Rin make money/support himself? **A** : He takes money off of his victims and he pickpockets for extra cash… Naughty Rin.

 **Q** : Why was the search party called off so soon? **A** : All will be explained in due time.. *evil laugh*

* * *

Yukio walked down the hallways of the massive True Cross Academy. It was ten minutes past curfew, and he, the class representative, was setting a bad example. Thankfully, no one was awake to see his offense. He'd fallen asleep in the library. Trudging back to the boys dormitory, he thought about possibly giving the young students a day off. They had been working extremely well and were showing massive signs of improvement.

He moved into the boy's dormitory after Rin went missing. He stayed with Bon, Shima and Konekomaru. There was no reason to stay in the abandoned dormitory now that his brother was gone.

As he was walking down a long stretch of hallways, he heard muffled voices. He slowed down to survey his surroundings. He was right outside of Mephisto's office. Quietly, he creeped over to the door and heard a heated conversation.

"-e has been killing my men! If we don't take action and kill him now he mig-"

"You will not harm him. Regardless of what you may think, he does not belong to you."

"I don't care who anyone thinks he is. He killed my men. He deserves nothing less than being executed."

"Arthur, as much as I love talking to you, this conversation is no longer private."

The door that Yukio had pressed his ear into began to swing open. He quickly pulled away to reveal a fuming Arthur sitting across from none other than Mephisto.

He quickly apologized "I apologize, Sir, I had no intention of being intrusive."

"Now, now, Okumura, There is no need for that. Arthur was just leaving. Weren't you, Arthur?" He purred.

The Paladin gaped at him, opened and closed his mouth a few times as though thinking about a snarky remark, and finally decided against it.

Standing up, he shoved past Yukio. Before he could leave, Mephisto said "Oh, and, Arthur, my word on the situation is final."

Yukio watched as the man walked down the hall, take a right down a flight of stairs, and disappeared from his line of site. Feeling unsure if he too should leave, he muttered "Uh, Sir, I think I should retire for the night."

The headmaster smirked and cheerily said "Of course, of course! I think that would be best. That reminds me, the exwires will be joining you tomorrow."

Yukio nodded. He had already been informed that they would be coming along to observe.

"Now, I just wanted to say that you won't be in any danger with tomorrow's mission. Your job is simply to guide and protect the exwires, should anything go wrong." His signature smirk was unmoving, and quite unnerving.

"Ah, but I'm rambling. You have a big day ahead of you tomorrow. Hurry off to bed now. Tut tut~"

Yukio backed out of the room, closing the door behind him as he left the headmasters office. He wasn't exactly sure if he was more reassured or troubled about what the headmaster had said.

That night he fell into a dreamless sleep.

* * *

Rin shot up out of his cot. His heart was pounding hard against his chest. Wiping sweat from his brow, he sighed, and held his face in his hands.

Dreams were no longer sweet; instead, they were filled with nightmares.

Light sheets of rain tapping against his window threatened to lull him back to sleep, back into the mercy and the torment of his dreams.

Propping himself up on his elbows, he glanced up at the clock on his wall. The time on the clock filled him with hearty annoyance: 3:16am. Groaning, he threw the blanket off himself. He was awake far too early, but his chances of falling asleep again were incredibly slim. He stood up and meandered aimlessly around his apartment, briefly stopping to stretch his stiff shoulders.

After the women ran into traffic head on, he was forced to find a meal elsewhere. Luckily, it wasn't very hard.

A man who was contemplating suicide was standing atop a six-story-tall building. Sneaking up behind him, Rin used a small Swiss Army knife to slit his throat. The man got the out he wanted and Rin got his meal. He had tasted fair enough, though not nearly as sweet Rin would have liked. Suicide rates in Japan were fairly high and missing persons reports were higher. The man would not be missed. Even if he was, it meant nothing to Rin.

Though Rin healing abilities were impressive, scaling the side of a six story building was bound to leave a hint of stiffness behind. By mid-morning it would disappear completely.

Thirsty, he walked over to his fridge and took a peak at what it contained. Several labeled containers: Hearts, Fingers, Tendons, and many others marked with similar body parts. It should last him through the week. Pushing aside a container marked as Tongues, he reached the coffee grounds. Coffee was the only non-human thing that was still tolerable in taste.

While he was setting the kettle on, the sound of footsteps began echoing from the hallway.

He stood rigid; his body was completely frozen, as though he was a statue carved from stone.

The footsteps suddenly stopped. In its place, an eerie silence choked the house its vice grip.

The rain, which had been so delicately tapping the widows, was no longer present.

His ears perked, hearing the faint, unmistakable sound of voices. He tiptoed to his room and silently snagged Kurikara from the side of his cot.

Holding the sheath in one hand and the hilt in the other, he quietly crept towards the noise.

He stalked down the of the hall, listening for where the voices were loudest. There was no intelligible conversation, just harsh whispers. Standing by the door to his bathroom, he pressed his ear to the door. The voices were loudest there. He let go of Kurikara's hilt to grip the door handle. Hasty throwing the door open resulted in a loud bang as it hit the back wall of the bathroom.

Nobody was there. The whispers had stopped.

Stepping forward, he threw open the shower curtain, revealing nothing. The only entrance to the bathroom was through the door that he had entered. There wasn't a window that could be used to sneak in. His eyes scanned the room. He had heard someone. He heard something.

Lowering his sword that remained in its scabbard, he stepped back. He did a brief check of the rest of the apartment, but the result was the same.

He set his sword by his cot and reentered the kitchen, sitting down with a heavy huff. He knew he heard something. The footsteps, the voices, he heard them!

Turning off the kettle, he went back to his bedroom. Coffee no longer seemed remarkably appealing.

Rin tossed and turned restlessly when he retired to his cot. Eventually, sleep's icy grip latched onto him once more.

The clock on the wall ticked and now read 3:37am.

Deep down, Rin knew that there had been no one else in the apartment that night.


End file.
